This invention relates to test fixtures used in load pull testing of RF transistors in microwave frequencies (see ref. 1 and 2
Active RF components (transistors—DUT) need to be thoroughly tested at the operation frequencies before used in amplifier and other circuit designs. “Load pull” and “Source pull” are test methods which use impedance tuners to systematically characterize the DUTs under various load and source impedance conditions. Load pull or source pull are automated measurement techniques used to measure Gain, Power, Efficiency and other characteristics of the DUT, employing source and load impedance tuners and other test equipment, such as signal sources, directional couplers, test fixtures to house the DUT (device under test, typically an RF transistor) directional couplers and signal analyzers (FIG. 1, see ref. 3).
At high power the semiconductor DUT's become “non-linear”, i.e. input and output signals are related, but not any more directly proportional. A single frequency (Fo) sinusoidal signal at the input of the DUT is distorted at the output, meaning that, when exiting the DUT, it contains “harmonic components” (FIG. 11). Periodical signals, sinusoidal or not, can be described as a Fourier series of the fundamental (Fo) and harmonic (N*Fo) frequency components (see ref. 5). The wave-form of the exiting signal depends on the operation conditions of the transistor, especially the impedance of the load.008 The trajectory of the voltage across the IV (current-voltage) characteristic of the transistor is called the “load-line” and depends strongly on the load impedance, which is controlled by the impedance tuners. The load-line determines the efficiency, linearity and reliability of the transistor and amplifier operation. This is the main reason why the signal waveforms must be detected and optimized. In order to be able to observe the actual signal waveform at the DUT terminals in the time domain, whereas the measurement occurs at a different position in the network, we must transform the signals from the measurement reference plane to the DUT reference plane. For this we must work in the “frequency domain”; i.e. we must convert the time function f(t) into a Fourier series of harmonic signals, using the fundamental and harmonic components generated by the Fourier transformation (eq. 1) and apply the reference plane transformation frequency by frequency first. An inverse Fourier transformation allows then transferring the signal representation back from the frequency domain to the time domain:
                              f          ⁡                      (            t            )                          =                                            a              0                        2                    +                                    ∑                              k                =                1                            ∞                        ⁢                          (                                                                    a                    k                                    ⁢                  cos                  ⁢                                                            k                      ⁢                                                                                          ⁢                      π                      ⁢                                                                                          ⁢                      t                                        λ                                                  +                                                      b                    k                                    ⁢                  sin                  ⁢                                                            k                      ⁢                                                                                          ⁢                      π                      ⁢                                                                                          ⁢                      t                                        λ                                                              )                                                          (                  eq          .                                          ⁢          1                )            
Hereby f(t) is the original time function (in the case of a transistor DUT, these are the voltages V1(t), V2(t) and currents I1(t) and I2(t) at the input and output terminals (ports) of the transistor correspondingly, a(t) and b(t) are the injected and extracted (reflected) signal waves and ak and bk their amplitudes at the harmonic frequencies k*Fo and λ is the wavelength at the given fundamental frequency (λ(mm)≈300/F(GHz)), wherein F is the frequency. To be able to measure the real shape of the non-sinusoidal signals V(t) and I(t), which we define as “wave measurement” we must measure in the frequency domain the amplitude and phase of the fundamental and harmonic frequency components and convert back to the time domain. To do so we must detect those frequency components using signal samplers also called directional couplers (FIG. 1) and measure the frequency components using appropriate signal analyzers.
It is obvious (FIG. 11) that the higher the number of harmonics considered, the more accurate the description of the time behavior of the original signal becomes. This means, however, that as the harmonic components traverse the network and, because all networks have “low pass” behavior 110 to 111 shapes in FIG. 11, higher harmonic components will be suppressed more than lower ones (the signal is “smoothed”). In other words, the form of a non-sinusoidal signal changes as it travels along the “low pass” transmission line. By transforming back (“embedding”) the signal harmonic components, measured by the signal analyzer, with the transfer matrix between the measurement point and the DUT, will reconstitute the original signal form. However parasitic components and strong low pass behavior of the network reduce the higher harmonic components over-proportionally (FIG. 11); then reconstitution of the original signal form is affected negatively. True reconstitution requires very high accuracy both of the measurement at the deferred position and the transfer matrix of the transformation section (35) in FIG. 3, respectively (71, 72) in FIG. 7, between the measurement position and the DUT terminals. This is the typical problem in analog telephony and associated distortion of the higher tones (harmonics) in voice or music transmission. Therefore, the closer the actual measurement reference plane to the DUT is, the easier and more accurate will be the reconstitution of the original signal waveform.
At high frequencies most electronic equipment, such as signal and network analyzers, operate in the frequency domain. In the frequency domain it is also easy to shift the reference plane of the measurement (em- and de-embedding). In order to sample the signal's components over a wide frequency range (the more harmonic components are known, the more accurate is the correspondence between the time and frequency domain of a signal, see FIG. 4) we need wideband signal directional couplers. Those couplers can be connected on both sides of the test fixture in which the DUT is mounted (FIG. 1). However such a setup includes connectors, adapters and transmission lines between the DUT and the couplers and suffers from higher insertion loss and low pass behaviour and creates considerable signal deformation due to parasitic components of the connectors and the other fixture parts. This causes the necessary reference plane corrections to become larger and thus possibly inaccurate.